1. Field of the Invention
The present invention relates to a light-activated semiconductor-controlled rectifier device which is turned on by a photo signal.
2. Description of the Prior Art
The light-activated semiconductor-controlled rectifier device (hereinafter referred to as the photo thyristor) having such a function as to be switched from the forward-blocked state to a conducting state by the radiation of light, like the ordinary electric gate type thyristor, comprises at least a semiconductor substrate with four continuous layers of PNPN and a couple of main electrodes in ohmic contact with the two outer layers of the semiconductor substrate. This photo thyristor is different from the electric gate type thyristor in that in the electric gate type thyristor a trigger current is supplied from the gate electrode electrically connected to, say, one of the intermediate layers, whereas the photo thyristor is supplied with a trigger current in the form of the photo current generated in the semiconductor substrate by the radiation of light from a light source electrically insulated from the semiconductor substrate. In order to improve the firing sensitivity of the photo thyristor, it is necessary to provide a construction whereby the photo current is generated and utilized efficiently. The conventional photo thyristor, especially, a device of comparatively large capacity, has the disadvantage that the photo firing sensitivity is reduced due to the limitations on the forward voltage increasing rate dv/dt and the breakdown voltage at high temperatures. This fact will be explained more in detail below with reference to the accompanying drawings. A prior art type photo thyristor is shown in FIG. 1 and FIG. 2. A semiconductor substrate 1 comprises four layers including a P-type emitter layer P.sub.E, an N-type base layer N.sub.B, a P-type base layer P.sub.B and an N-type emitter layer N.sub.E successively arranged between a pair of main surfaces 11 and 12 located on opposite sides of the semiconductor substrate 1. The P.sub.B layer is exposed to the other main surface 12 through a multiplicity of short-circuiting apertures 2 scattered over the entire area of the N.sub.E layer and through substantially the central portion of the N.sub.E layer. Reference symbols J.sub.1, J.sub.2 and J.sub.3 show first, second and third PN junctions formed between P.sub.E layer and N.sub.B layer, between N.sub.B layer and P.sub.B layer, and between P.sub.B layer and N.sub.E layer respectively. Numeral 3 shows an anode in ohmic contact with the P.sub.E layer on the one main surface 11, numeral 4 a cathode provided on the other main surface 12 which is in ohmic contact with the N.sub.E layer and the P.sub.B layer exposed by way of the short-circuiting apertures 2, and numeral 5 means for radiating a photo signal. When light is radiated from the photo signal radiating means on the surface of the P.sub.B layer exposed to the other main surface 12 through substantially the central portion of the N.sub.E layer with a forward voltage applied between the electrodes of the photo thyristor in such a manner as to render the anode 3 positive with respect to the cathode 4, a photo current is generated in the P.sub.B layer. This photo current flows toward the short-circuiting apertures 2 nearest to the portion radiated with light, as shown by arrows in the drawings, and by way of the short-circuting apertures 2 reaches the cathode 4. This photo current forward-biases the innermost periphery of the third PN junction J.sub.3 of the N.sub.E layer and the P.sub.B layer, where electrons are injected from the N.sub.E layer into the P.sub.B layer, so that the photo thyristor is switched from the forward blocked state to a conducting state and becomes to be turned on. If the inner radius of the N.sub.E layer, namely, the radius R.sub.1 of the portion of the P.sub.B layer which is exposed through substantially the central portion of the N.sub.E layer is enlarged, the photo current generated by the radiation of the photo signal is dispersed among the peripheral short-circuiting apertures and therefore the density of the photo current for forward biasing the innermost periphery of the third PN junction J.sub.3 is reduced, with the result that a great amount of photo current is required for turning on the device, thereby reducing the firing sensitivity thereof. On the contrary, if the value R.sub.1 is reduced, the part of the N.sub.E layer on the path of the photo current is lengthened and therefore the innermost side of the third PN junction J.sub.3 is strongly biased forwardly with a relatively small amount of photo current, resulting in an increased firing sensitivity. Since in this case the innermost peripheral side of the third PN junction J.sub.3 is easily forward-biased by the displacement current and the leakage current of the second PN junction J.sub.2, however, the disadvantage is that the critical increasing rate of applied voltage, namely, the dv/dt capability and the breakdown voltage at high temperatures are decreased. In order for the innermost peripheral side of the third PN junction J.sub.3 to be easily forward biased without any reduction in the dv/dt capability or the breakdown voltage at high temperatures, the short-circuiting apertures 2 on the innermost peripheral side should be relocated inwardly toward the center to such an extent as to achieve a desired dv/dt capability, together with the reduction of radius R.sub.1. In this case, with the relocation of the short-circuiting apertures 2, the inner edge of the cathode 4 changes its position toward the center of the device, so that the light-radiated surface area is reduced thereby decreasing the amount of photo current generated, resulting in a relative reduction in the firing sensitivity.
As will be noted from the foregoing description, it has so far been very difficult to obtain a photo thyristor having a high firing sensitivity as well as high dt/dv capability and high breakdown voltage at high temperatures.